The prior invention relates to devices for operating the gear change of a bicycle. In general, this device is a lever type shifter, and the user may push the lever of the shifter for winding the cable of the derailleur to tighten or relax it. An index device is used for telling the extension of the cable subject to different shifts so as to drive the rear derailleur for the change to each gear.
In the course of gear change of the derailleur, because of the lash between the gear and the chain and the relaxation of the cable, errors in the derailleur shift would occur so that the derailleur gear drives the chain to move another distance of stroke in order to allow the chain to set into the right shift of the gear cluster. Then, the derailleur gear will return to the right position. The extra stroke necessary for keeping the shifting correct is called "over shift," which may prevent the derailleur from being affected by errors, while keeping the operation correct as well as ensuring the operation to be correct for as long as it is used.
The major defects of the lever type shifter include:
1. Complexity of the operation--the rider has to push the shifter with his or her thumb and index finger in order to change gears while he or she is paying attention to steering the bicycle and the brake function. Manipulation and safety of riding has thus been reduced;
2. Low efficiency of the cable--because the cable is winding around the disc, its transmission efficiency would be reduced to cause labor waste for operation due to stress concentration, fatigue destruction, and arc winding;
3. To improve the aforesaid operating problems, some of the shifters have adopted ratchet mechanisms for governing the shift, but that increases the number of parts, the cost, and assembly work load;
4. To attain the purpose of the over shift, the shifter has to add an over shift device.
Referring to FIG. 1, the conventional rotary shifter includes a fixing seat 1 mounted on the handlebar of a bicycle, and a rotary lever 2 with an identical shaft center to the handlebar for the rider to hold and manipulate with his or her palm. With the rotation of lever 2, it drives the derailleur cable 3 to drive the derailleur to change the shift. The fixing seat 1 and lever 2 can be mounted on the handlebar near the area between the thumb and the index finger of the rider so that the rider may hold the handlebar and the lever 2. The rider may then change the gear without leaving his or her palm from the handlebar. However, the action of the conventional rotary shifter remains for using the winding way to drive the cable so that the operating efficiency is not high, and besides, the cable is liable to fatigue and fracture. In addition, the derailleur cable 3 of such type has to be specially winding around the actuator, which is very difficult to assemble. Further, to attain the purpose of over shift, the conventional rotary shifter requires an over shift device.